Field of the Invention
The invention relates to a power switching device having a power switching transistor connected in series in a load circuit with an inductive load portion, and having a commutation circuit, which is connected in parallel with the gate-drain or base-collector path of the transistor and has at least one Zener diode, which diode determines the clamping voltage of the power switching transistor during commutation, and a normal diode connected in series and in a manner oppositely biased with respect to the Zener diode.
Typical areas of application for power switching devices are, inter alia, synchronous rectifiers and switching regulators. In this case, such power switching devices, which have either a bipolar power switching transistor or a field-effect power switching transistor, are realized either in a so-called low-side configuration, where the power switching device is located on the side of the load with lower potential, or in a so-called high-side configuration, in which the power switching device is located on the side of the load with higher potential.
The increased integration density in integrated circuits leads, in MOS power field-effect transistors as well, to smaller chip areas for the same on resistance. This results, inter alia, in a reduction of the maximum energy strength during the commutation operation in load circuits with inductive loads.
The accompanying FIGS. 4 to 6 illustrate the prior art of power switching devices respectively on the basis of a low-side configuration and a high-side configuration. The power switching transistor T1, by way of example, is an NMOS field-effect transistor in this case. The power switching transistor T1 is connected in series with an inductive load L1 in a load circuit, to be precise in a low-side configuration in FIG. 4 and in a high-side configuration in FIG. 5. A current IL1 flows between a supply voltage Vbb and ground in the switched-on state of the power switching transistor T1. The current leads to a voltage UL1 across the load L1. The voltage is shown in FIG. 6. The series circuit which comprises a Zener diode D1 and a normal diode D2 connected in a manner oppositely biased with respect thereto and is connected in parallel with the gate-drain diode of the power switching transistor T1 forms a commutation circuit, wherein the Zener diode D1 determines a clamping voltage UKL (FIG. 6), at which the transistor T1 is turned on during commutation. The Zener value of the Zener diode D1 is determined by the maximum operating voltage at Vbb at which the transistor T1 is permitted not to be turned on. In the switched-on state, the diode D2 ensures that the gate-drain diode path is blocked. The commutation energy converted during the switch-off of the inductive load L1 in the power switching transistor T1 results from the value of the load current IL1 integrated over time multiplied by the voltage drop across the power switching transistor T1. This commutation energy leads to the heating of the power switching transistor T1, the maximum possible energy value resulting from the maximum possible destruction temperature of the power switching transistor T1 and the absolute material volume (of the power switching transistor) in which the energy is converted.